High speed image sensors have been widely used in many applications in different fields including the automotive field, the machine vision field, and the field of professional video photography. The technology used to manufacture image sensors, and in particular, complementary-metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demand of higher frame rates and lower power consumption has encouraged the further miniaturization and integration of these image sensors.
CMOS image sensors have to take into account the dark current on photo diodes or floating diffusion of pixels. The dark current appears as dark signal when the image signal from a pixel array is readout. In current image sensor systems, the summation of the signal and dark signal are readout at the same time. Thus, when processing the image signal, there is an ADC time for the dark signal.
Current contact image sensors (CIS) often have optical black pixels read out dark signals. The dark signals read out from the optical black pixels are subtracted from the output signal of the visible pixel (or visible signal) to extract the real signal. Prior art solutions to subtract the dark signal from the visible signal include (i) subtracting dark signal in the digital domain and (ii) subtracting dark signal in the analog domain.
In one prior art, after ADC, the dark signal is subtracted from the visible pixel signal output such that there is a need to first convert from analog to digital the sum of the dark signal and the visible signal. In another prior art, the dark signal is subtracted from the visible signal in the analog domain before the ADC. In this prior art, while there is no ADC time for the dark signal, there is a need for special analog circuits to subtract the dark signal. These analog circuits create a requirement of space and power and often worsen horizontal random noise. Further, the analog circuits are not sufficiently precise such that digital black level correction (BLC) is needed in addition to the analog BLC.
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